The first bimetallic imidazolates containing alkali and alkaline earth metals, NaMgIm(3) and KMglm(3), respectively, are prepared by mechanochemical synthesis and are reported in this paper. NaMgIm(3) has been prepared by the reaction between NaIm and Mg(BH4)(2) as well as directly from NaIm and MgIm(2). Structural evolution and thermal stability were followed by an in situ high-temperature X-ray powder diffraction experiment utilizing synchrotron radiation. In both compounds, the imidazolate ligand is connected to four metal cations forming a complex three-dimensional network with channels running along the c-direction. NaMgIm(3) and KMgIm(3) are the first members of a new family of imidazolate frameworks with stp topology. The formation of mixed-alkali-metal imidazolate compounds is thermodynamically controlled. LiIm and MgIm(2) have not yielded a mixed-metal compound, while KIm reacts swiftly and forms KMgIm(3).